Expandable polymers



United States Patent 3,370,022 EXPANDABLE POLYMERS Alvin R. Ingram,Murrysville, and Alvin J. Zupanc, Irwin, Pa., assignors to KoppersCompany, Inc., a corporation of Delaware No Drawing. Filed Dec. 3, 1965,Ser. No. 511,319 6 Claims. (Cl. 260-25) This invention relates generallyto expandable thermoplastic material and more particularly improvedexpandable styrene polymer particles which exhibit a strong anti-lumpingproperty during preexpansion.

Expandable particles or beads of thermoplastic material generallycontain a blowing agent which will boil below the softening point of thethermoplastic material and which will cause the particles to expandunder the influence of heat. conventionally, the particles are heated byinjecting steam or other suitable heat media under pressure into themold cavity to heat the particles above their softening point whereuponthe particles expand to fill the mold cavity and fuse together. Theparticles can be either placed directly into the mold or they canpreexpand before being placed in the mold as, for example, is describedin United States Patent No. 3,023,175.

While thermoplastic expandable polymer particles have worked well toform a wide variety of articles, they have had certain disadvantages.During the preexpansion process, the particles have a tendency to sticktogether and form lumps which renders the particles unsuitable formolding. Heretofore, to alleviate the lumping problem, a lubricant hasbeen added to the surface of the particles prior to preexpansion suchas, for example, silicone oil or magnesium stearate, but the amount oflubricant had to be carefully controlled. In fact, it has been foundthat satisfactory fusion of the particles on molding is not achievedwhen lubricants are added in slight excess of 7 that needed. The use ofa lubricant has other disadvantages, for example, silicone oil has theeffect of reducing the shelf life of the particles due to a rapid lossof blowing agent and also impairs the free-flowing properties of thebeads because of its oily nature. Solid lubricants such as magnesiumstearate have the disadvantage of being difficult to distribute evenlyon the beads.

Surprisingly, we have found that the expandable polymer particles can bemade free from lumping during preexpansion by the addition ofunsaturated polyesters to the polymerizing monomer or mixtures thereofat the proper stage of the polymerization. Excessive amounts do notimpair free flowing of the beads, the shelf life or the fusion of thefoam.

In accordance with this invention a vinyl aromatic monomer either aloneor in admixture with other polymerizable monomer copolymerizabletherewith is suspended in water with the aid of a suspending agent andpolymerized in an aqueous suspension in the presence of a catalyticamount of a free radical initiator until about between 50 and 95 percentconversion of the vinyl monomer to polymer has occurred. Then, basedupon the weight of monomer, there is added at least about 0.05 percentby weight of an unsaturated polyester and the polymerization iscompleted, The polymer particles are rendered expandable byincorporating therein a volatile organic blowing agent.

Polymerizing monomers in an aqueous suspension is a well-knownoperation. The monomer or mixture of monomers, to which has been added afree radical initiator, in the form of an oil phase is admixed withwater and heated.

Typical free radical initiators are oil soluble free radical catalystssuch as benzoyl peroxide, t-butyl perbenzoate, lauroyl peroxide,azo-bis-isobutyronitrile, etc.

The mixture is stirred or agitated so that the oil phase is in the formof small individual droplets.

Patented Feb. 20, 1968 The temperature for the polymerization, ofcourse, must be high enough so that the polymerization occurs at areasonable rate depending upon the catalyst system used with thepreferred temperature range being between about and 120 C.

After the polymerization has progressed to a certain point, which variesdepending upon the nature of the monomer or monomers, the droplets tendto become sticky so that stirring alone is insufficient to keep thedroplets from coalescing. It is therefore necessary to have suspendingor stabilizing agent present in the polymerization mixture. Examples ofsuitable stabilizing agents are polyvinyl alcohol, hydroxyethylcellulose, tricalcium phosphate, etc.

In accordance with this invention when the polymerization has reached apoint between about 50 and 95 percent conversion of the vinyl monomer,at least 0.05 percent based upon the weight of monomer of an unsaturatedpolyester resin is added to the suspension. Quantities of polyesterresin less than 0.05 have no appreciable elfect in reducing the lumpingtendency of the particles. On the other hand as a practical matterquantities of polyester resin of over 2.0 percent have generally beenfound to be unnecessary and wasteful. Generally, the earlier thepolyester is added to the suspension of polymerizing monomer, the lesspolyester is needed to achieve the desired anti-lumping properties.

The time of addition of the polyester is critical and should be madebetween about 50 and 95 percent conversion. It has been found thataddition of the polyester to the monomers prior to polymerizationresults in very unstable suspensions. Addition prior to about 50 percentconversion gives particles having very odd shapes. The product particlesare disc-like when the addition is made from about 50 to percentconversion with the shape becoming more spherical at the higherconversions. Addition of the polyester between about 75 and percentconversion results in predominantly round beads. Addition after 95percent conversion while not affecting the shape of the beads iswasteful since only a small portion of the polyester will react with thealmost completely polymeriZ/ed monomer and large amounts of polyesterare needed in order to provide a satisfactory anti-lumping property,While, if it is desired to produce spherical granules, the additionshould be made to correspond with between 75 and 95 percent conversionof the monomer, earlier addition of the polyester (i.e., between about50 and 75 percent conversion) is not a disadvantage if particles of asomewhat flatter shape are desired since the particles give equally goodresults with regard to antilumping and molding properties and have theadvantage of not being a hazard to operating personnel should they bespilled on the floor during subsequent operations.

The unsaturated polyesters react with the polymerizing monomer to crosslink the surface of the polymer particles. This is evidenced by theformation of toluene insoluble material. The harder cross-linked surfacehas the effect of preventing individual particles from fusing togetherto form lumps or agglomerates during the early stages of preexpansion.However, after preexpansion has occurred, the surface is sufficientlyenlarged so that the cross-linked portions are not sufficientlyconcentrated to have any deleterious affect upon the fusion propertieswhen the particles are molded. Furthermore, due to the fact that thecross-linking polyesters are in themselves long chain ploymerscross-links which are provided are flexible rather than rigid so thatexpandability is not adversely affected. In fact the presence of thepolyester increases the flexibility of the polymer foam when it ismolded into thin walled articles, such as cups. The lips of the cups canbe squeezed together without fracturing the cups.

The unsaturated polyesters useful in the practice of our invention arethose prepared by condensing dihydroxy alcohols with unsaturated dibasicacids, for example, the polymerization products of glycols such aspropylene glycol with unsaturated dibasic acid anhydrides such as maleicanhydride with or without saturated dibasic anhydrides such as phthalicanhydride. The polyesters can be further modified by monocarboxylicacids, monohydroxy alcohols and small amounts of polycarboxylic acidsand polyhydroxy alcohols. The polyesters are employed in the form of aviscous liquid resin formulation prepared by mixing the resin with avinyl monomer such as styrene or diallyl phthalate in amounts of about2540 percent by weight of monomer based on total weight of formulation.Such formulations are sold under the trademark Koplac (Koppers Company,Inc.).

In order to evenly disperse the unsaturated polyesters on the surface ofthe beads, it has been found advantageous to dissolve a small amount ofa surface active agent in the polyester resin formulation prior to thetime that it is added to the suspension. Suitable active surface agentsinclude those normally used during the impregnation of the polymerparticles with blowing agent, i.e., polyoxyethylene sorbitanmonolaurate; amounts of about 0.1 or 0.2 percent of surface activeagents have been found effective for this purpose based on monomer.Since the unsaturated polyester resin formulations are somewhat viscousit has also been found convenient to further dilute the resinformulation with styrene monomer which not only aids in theincorporation of the dispersing agent in the resin but also improves thedistribution of the resin on the surface of the polymer particles. Theamount of styrene employed is generally in a ratio of 1:4-1z1 by weightstyrene to polyester resin formulation.

The blowing agent may be incorporated in the polymer particles at thedesired time, for example, during polymerization or after thepolymerization has been completed. Suitable blowing agents can beemployed which boil below the softening point of the polymer, forexample, aliphatic hydrocarbons such as pentane, isopentane, hexane,cyclohexane; halogenated aliphatic hydrocarbons such as ethyl chloride,propylchloride, isopropylbromide, isopropylchloride,dichlorodifluromethane, trichlorofluoromethane,1,1,2-trichlorotrifiuoroethane, etc. The blowing agents are employed inamounts of from 320 percent by wei-ght based on the monomer charge.

Our invention is further illustrated by, but not limited to, thefollowing examples in which parts are parts by weight unless otherwiseindicated.

Example I To a 5 gallon glass lined kettle equipped with a threebladedimpellor, there was charged consecutively 100 parts of styrene monomer,0.45 part of a catalyst mixture comprising 0.30 part'benzoyl peroxideand 0.15 part t-butyl perbenzoate, 102 parts distilled water and 0.10part a buffer tetrasodium pyrophosphate. The mixture was stirred by thethree-bladed impellor at a speed of 200 r.p.m., heated to 90 C. andmaintained at that temperature and approximately 88 minutes afterreaching 90 C., 0.175 part of suspending agent hydroxyethyl cellulose(Natrosol 250-G-R, Hercules Powder Co.) was added. After the mixture hadbeen heated at 90 for a total of 4.75 hours, when it was determined bysample that approximately 75 percent of the monomer had been convertedto polymer, there was added 1.0 part of Koplac 3700-50 (1:1 (mole basis)propylene glycolmaleic anhydride polyester in 28 percent by weightstyrene) containing 0.2 part of the dispersing agent Tween(polyoxyethylene sorbitan monolaurate). After the addition of thepolyester, heating at 90 C. was continued for 1 hour and then 8.5 partsof the blowing agent n-pentane were added to the suspension over a 90minute period. The temperature of the suspension was then raised to 115C. which took minutes and maintained at 115 C. for an additional 4 hoursto complete the polymerization and impregnate the blowing agent into theparticles. The polymerization slurry was dewatered and the beads rinsedwith water after which the wet beads were dried in an air stream at roomtemperature for about 3 hours. To determine the degree of cross linking,a 1 gram portion of the product beads was mixed with 100 milliliters oftoluene at 25 C. The toluene was filtered through a tared sintered glassfilter and the filter dried for 12 hours at 75 C. The filter wasreweighted to determine the amount of polymer retained on the filter.The toluene insoluble portion was 0.3 percent by weight.

The product beads were then separated into two portions by size PortionA, through 10 and On 25 mesh U.S. Standard Sieve Series, and Portion B,throuh 25 and on mesh U.S. Standard Sieve Series. The volatile content,density on preexpansion, and lumping on preexpansion were determined oneach portion of the beads as follows: one gram samples of each portionwere accurately weighed in an aluminum weighing dish on an analyticalbalance and then heated for 2 hours in an oven at 150 C. After the beadshad been cooled to room temperature in a desiccator, the weighing dishand contents were reweighed on the balance. The weight loss wascalculated and reported as the percent volatile content of the beadsample. Portion A had a volatile content of 6.26 percent by weight andPortion B had a volatile content of 5.62 percent by weight. Anexpandability and lumping test was conducted on each portion of thebeads by placing 4 gram quantities of the portions separately into acylindrical screen cage (4" high x 3.5" in diameter) along with two No.2 rubber stopper tumblers. To preexpand the heads, the cage was rotatedfor 3 minutes at 60 rpm. in a closed container in a steam atmosphere atatmospheric pressure. The expanded beads were recovered from the screencage and allowed to air dry in paper containers for approximately 18hours. The aged, expanded beads were screened through a No. 3 /2" meshU.S. Standard Sieve and the percent lumping determined from the weightof the beads which were retained on the screen. Portion A beads had 2percent lumps and Portion B beads had 11 percent lumps. The bulk densityof the Portion A beads was 1.47 lbs./ cu. ft. and the bulk density ofthe Portion B mesh beads was 1.69 lbs./ cu. ft.

The -25 +40 mesh beads (Portion B) were expanded to a density of 4.5lbs/ft. and then aged for 24 hours. The foam particles were placed in acup-shaped mold cavity thickness x 2%" diameter at the top and 2diameter at the bottom) surrounded by a steam jacket. Steam was injectedinto the mold cavity to cause the beads to expand and fuse together. Thecup molded from the beads was well fused and of an attractive appearanceand of such a flexibility that the lips of the cup were squeezedtogether 15 times without fracturing the cup. In contrast, a cup moldedfrom a commercial product (Dylite F-40) fractured after 5 times whenthis is attempted.

Beads were prepared by the above procedure but without adding thepolyester resin. The beads exhibited 40 and 75 percent by weight lumpsin the 'l0 to +25 mesh and -25 +40 mesh portions respectively.

Example 11 To a 2-liter resin kettle equipped with a three-bladedimpellor there was added 100 parts of the monomer styrene, 0.45 part ofa catalyst comprising 0.30 part benzoyl peroxide, 0.15 part t-butylperbenzoate, 100 parts of water and 0.1 part of the buffer, tetrasodiumpyrophosphate. The mixture was agitated at 200 rpm. and heated to 9092C. After about minutes at 92 C., 0.2 part of the suspending agent,hydroxyethyl cellulose, was added and the stirring and heating at 92 C.was continued for an additional 4 hours and 40 minutes after which theslurry was cooled at 23 C. To individual beverage bottles were addedgrams of the reaction slurry, 40 milliliters of water, x grams of thepolyester resin listed in Table I containing 0.1 x grams ofpolyoxyethylene sorbitan monolaurate dissolved therein, additional (0.16gram-0.1 x grams) polyoxyethylene sorbitan monolaurate (where x is theamount in grams of the polyester It can be seen from the results shownin Table II that the addition of the polyester to the monomer resultedin failure of the suspension. Addition prior to the bead identity pointresulted in the preparation of particles ranging resin needed to givethe percent by weight of polyester 5 in shape from very irregularparticles when the addition resin based on styrene monomer which isshown in Table is made at 1.4 hours (39 percent conversion) to flatdiscs I) and 11.5 milliliters of the blowing, agent, n-pentane. when theaddition was made at 2.5 and 3.0 hours (50 The bottles were then cappedand agitated end-over-end and 62 percent conversion) and thick discs at3.75 hours in an oil bath at 115 C. for four hours to complete the (72percent conversion). Addition at the bead identity polymerization and toimpregnate the blowing agent into 10 point or at 4.5 hours (about 81percent conversion) rethe beads. The beads were separated from thewater, sulted in round beads having good expansion and lumping rinsedwith water and air dried. The volatile content and characteristics.Addition after 6 hours where the beads density and lumping onpreexpansion were determined were hard (98 percent conversion) asevidenced by the as in Example I with the results shown in Table Ibelow. increased lumping tendency and the decreased amount TABLE IPercent by Volatiles, Density, Lumps, Experi- Polyester Resin Weight onPercent by 1bs./tt. 3 Percent by mant Styrene Weight Minutes in WeightSteam 8 1:1 (mole basis) propylene glycol-maleic anhydride in 28% byweight styrene. I b 1:0.5:0.5 (mole basis) propylene glycol-phthalieanhydrlde-maleic anhydride in 30% styrerie styrene.

The results, Table I, show the more reactive polyester resins (thosecontaining the higher percentage of maleic anhydride) gives a greateraitect on the lumping tendency of the beads since less of these resinsis required to give improved lumping properties.

Example III To demonstrate the importance of the time of the addition ofthe unsaturated polyester resin, a series of experiments was conducted.In each case, to a 5 gallon glass lined kettle there was chargedconsecutively 100 parts of the monomer styrene, 0.45 part of a catalystcomprising 0.3 part benzoyl peroxide, 0.15 part t-butyl perbenzoate, 102parts distilled water and 0.10 part of the buffer, tetrasodiumpyrophosphate. The mixture was stirred by a three-bladed impellor at aspeed of 200 rpm. and heated to 90 C. After 80 minutes at 90 C., 0.175part of the suspending agent hydroxyethyl cellulose was added. Heatingwas continued at 90 C. and the 0.5 part of Koplac 3700-50 polyesterresin containing dissolved therein 0.2 part of the dispersing agentpolyoxyethylene sorbitan monolaurate was added to the suspension at thetime shown in Table II. After the mixture had been heated a total of sixhours at 90 C., there was added 11.5 parts of the blowing agentn-pentane and the temperature was raised to 115 C. for four hours tocomplete polymerization and impregnate the blowing agent into theparticles. In the case of Experiment III-7 the blowing agent additionwas made after the polyester had been added. The percent by weighttoluene insolubles, expansion and lumping tests were conducted as inExample I and the bead size and shape was determined by screening andobservation. The results are shown in Table II.

0.67:0.33 (mole basis) propylene glyeol-phthalic anhydride-maleicanhydride in 38% of toluene insoluble material produced less effectivecross linking of the bead surface by the polyester due to a lower amountof styrene monomer being available for reaction. It can be seen that thetoluene insoluble materials (i.e., the amount of cross-linked material)decreases the later the polyester is added to the system.

. The portions of the beads prepared in Examples III- III, IIIIV andIII-V, which had been preexpanded for the lumping tests, were placed inindividual 5 x 5 x inch moldsand the molds placed between the platens ofa conduction press where the beads were heated to expand them and causethem to fuse together. The bead-tobead fusion in eachcase was excellent.

Example IV To illustrate the improvement achieved in the dispersion ofthe polyester resin on the surface of the bead by diluting the resinformulation with additional amounts of styrene and to show the optimumamount of dispersing agent to be added with the resin a series ofexperiments was conducted using the following procedure. To two-literresin kettles there was added 750 grams of the monomer, styrene,containing as catalysts 0.35 percent benzoyl peroxide and 0.15 percentt-butyl perbenzoate, 750 milliliters of water and 0.75 gram of thebuffer tetrasodium pyrophosphate. The kettles were heated to 90 C. andafter 82 minutes 1.3 grams of the suspending agent hydroxyethylcellulose was added. At four hours 1.5 grams of Koplac 3700-50 polyesterresin formulation containing the amounts of polyoxyethylene sorbitanmonolaurate (Tween 20) and additional styrene shown in Table III wasadded to the suspension and the heating at 90 C. continued for anadditional two hours to give a total heat- TABLE II Expansion in SteamTime of Toluene 3 minutes Experiment addition Appearance of Insolubles,Predominant Bead at 90 C Suspended Phase Percent by Size and Shape 1Density, Lumps,

(hrs.) Weight lbs/ft. Percent by Weight Suspension failed 1. 4 Liquid 2.5 Irregular particles"..- 1. 00 2 2. 5 Viscous 1. 2 10+20 mesh flat. .941 3. 0 Tacky viscous 8 .d0 0. 94 Nil 3. Highly viscous- 2 16+20 meshflat 0. 97 Nil 4. 5 Soft beads 12 16+20 mesh round 0. 94 Nil 6. 0 Hardbeads. 08 -20+24 mesh round 0. 96 3 1 US. Standard Sieve Series. 2 Tomonomer.

7 ing time at 90 C. of six hours. The suspension was then cooled to 23C. and two 200 gram portions of the slurry were placed in individualbeverage, bottles. To one bottle was added 12.5 milliliters of theblowing agent n-pentane, Without additional dispersing agent(Experiments IV1 to IV6). To the other bottle was added 12.5 millilitersof the blowing agent n-pentane and 0.1 gram of polyoxyethylene sorbitanmonolaurate (Experiments 1V1A to IV-6A). The bottles were capped andheated in an oil bath at 115 C. with end-over-end agitation for fourhours, after which the bottles were cooled, the product beads dewatered,rinsed with water and dried in a stream of air at room temperature fortwo hours. The expandable bead properties were determined as in ExampleI along with the approximate external cell size of the preexpandedbeads. The results are shown in Table III below. It has been foundempirically that beads that lump less than 11 percent in the laboratory,usually lump less than 1 percent in the Rodman preexpander (US. PatentN0. 3,023,175). In this case 0.2 percent Koplac 3700-50 polyester resindiluted with 0.1 percent styrene monomer eliminates lumps from the beadsif the amount of dispersing agent in the polyester resin is at least 0.1percent with or without additional dispersing agent being added justbefore the impregnation with blowing agenLToo little dispersing agent,.025.05 percent, with the resin does not-distribute the resin wellenough While excess dispersing agent may remove some of the resin fromthe bead surface and cause it to be emulsified in the aqueous phasealthough apparently the additional dispersing agent does favorably lowerthe external cell size giving a better appearance to the foam. Althoughsome beneficial effect on lumping can be noted by the dilution withincreasing amounts of styrene, its major advantage is to facilitate theincorporation of dispersing agent in the resin formulation by reducingthe viscosity of the formulation.

dried in an air stream for about 3 hours at room temperature. The beadshad a volatile content of 5.42 percent by weight, 1 percent by weightlumps, and a bulk density of 1.60 lbs/cu. ft. as determined by the testprocedures described in Example 1.

Example Vl To a 2500 gallon reactor there Was charged consecutively 100parts of the monomer styrene, 0.45 part of the catalyst comprising 030part benzoyl peroxide, 0.15 part t-butyl perbenzoate, 102 parts of waterand 0.10 part of the buifer tetrasodium pyrophosphate. The mixture wasstirred by a three-bladed impellor at a speed of 65 r.p.m., heated to 92C. and maintained at that temperature. Approximately 80 minutes afterreaching 92 C. 0.175 part of the suspending agent hydroxyethyl cellulosewas added and the impellor speed was increased to 80 r.p.m.Approximately 10 minutes after the hydroxyethyl cellulose addition wascompleted there was added 0.2 part of ethylene-bis-stearamide as a pastecontained in styrene (prepared by stirring up 0.4 part of amide with 1.5parts of styrene). After an additional 150 minutes at 92 C. there wasadded 0.4 part of Koplac 370050 polyester resin formulation, containing0.07 part of the dispersing agent polyoxyethylene sorbitan monolaurate(Tween 20) and an additional 0.1 part of styrene monomer. Heating wascontinued for an additional 120 minutes and then there was added 8.5parts of the blowing agent n-pentane which took 30 minutes. Thepolymerization was then completed and the beads impregnated with theblowing agent by raising the temperature to 115 for 4 hours. Thereafter,the mixture was cooled to room temperature and the particles removed,washed with water and air dried. Portions of the beads were tested forexpandability and lumping according to the procedure described inExample I. The beads had a bulk density of 1.00 lb./cu. ft.

TABLE 111 Added with 2% by Weight Extra Expandable Bead CharacteristicsKoplac 3700-50 Tween 20, Experi- Percent by Volatiles, Density, ExternalLumps,

ment Styrene, Tween 20, Weight at Percent by lbs/ft Cell Size, Percentby Percent by Percent by Impreg- Weight Minutes in Mils Weight WeightWeight nation Steam IV-l 0. 05 0. 5. 90 1. 22 3-4 7 0. 05 0. 10 0. 10 5.90 1. 24 2 10 0. 10 0.10 5. 91 1. 29 2-4 4 0. 10 0. 10 0. 10 5. 85 1. 262 10 0. 0. 10 5. 99 1.26 2-4 4 0. 20 0. 10 0. 10 5. 88 1. 26 2 10 0. 100. 075 6. 00 1. 22 4 5 0. 10 0. 075 0. 125 5. 96 1. 24 2 25 0. 10 0.0505. 95 1. 33 4 11 0. 10 0. 050 0. 15 5. 90 1. 26 2 30 0. 10 0.025 6.06 1.31 4 21 IV-6A 0. 10 0. 025 0. 175 5. 90 1. 24 2 38 1 Polyoxyethylenesorbitan monolaurate.

Example V To a 100 gallon reactor there was added 100 parts of themonomer styrene containing 0.40 part of the catalyst consisting of 0.25part benzoyl peroxide and 0.15 part t-butyl perbenzoate along with 73parts of water, 0.364 part of the suspending agent system comprising0.363 part tricalcium phosphate and 0.001 part Nacconol NRSF (dodecylbenzene sodium sulfonate). The slurry was stirred and heated to 90 C. atan agitator speed of 95 r.p.m. After 7 hours at 90 C. there was thenadded to the suspension 0.25 part Koplac 3700- polyester resin (1:1propylene glycol-maleic anhydride in 28% by weight styrene monomer)containing 0.10 part of the dispersant, Atlas G 2127 (polyoxyethylenelaurate). Heating was continued for 0.5 hour at 90 C. after which therewas added as additional dispersants, .15 part tricalcium phosphate and.01 part Nacconol NRSF and 8.5 parts n-pentane which took 0.5 hour. Theslurry temperature was then raised to 115 C. and heating continued for 4hours at 115 C. to complete the polymerization and to impregnate theblowing agent into the beads. The bead slurry was dewatered, rinsed withWater, and the wet beads and the percent lumping was nil. Portions ofthe preexpanded beads were then placedin a 20 x 20 x 12 inch mold cavitywhich was surrounded by a steam chest. Steam was injected into the moldfor 30 seconds to cause the beads to expand and fuse together. Thebead-to-bead fusion in the resulting foam blocks was good and the timerequired to cool the foam to a non-shrinking selfsustaining structurewas 7 minutes.

Example VII To a two-liter resin kettle there was added 100 parts of themonomer styrene containing 0.45 part of the catalyst comprising 0.3 partbenzoyl peroxide and 0.15 part t-butyl perbenzoate, 102 parts of water,0.10 part of the buffer, tetrasodium pyrophosphate, 2 percent based onstyrene of the self-extinguishing agent, tris-(2,3-bromopropyl)phosphate, and 0.35 percent by weight based on styrene of a peroxidesynergist, 2,5-t-butyl peroxy-2,5-dimethyl hexane. The mixture wasstirred by a three-bladed impellor at a speed of 200 r.p.m., heated to92 C. and maintained at that temperature. Approximately minutes afterreaching 92 C., 0.10 partof the suspending agent,

hydroxyethyl cellulose, was added. After an additional 152 minutes at 92C., there was added 0.5 part of Koplac 3700-50 polyester resincontaining 0.1 part of the dispersing agent, polyoxyethylene sorbitanmonolaurate, and heating was continued for an additional 60 minutes. A160 gram portion of the slurry which had been cooled at 23 C. was placedin a beverage bottle along with 0.1 gram of polyoxyethylene sorbitanmonolaurate and 10.5 milliliters of a blowing agent which was a 50:50 byvolume mixture of isopentane and n-pentane. The bottle was capped androtated end-over-end in an oil bath heated to 115 C. for 240 minutesafter which the bottle was cooled and the beads separated from theaqueous phase by a centrifuge and washed with water and air dried. Thebeads had a volatile content of 5.97 percent by weight, expanded to abulk density of 0.99 lbs/cu. ft. and had 3 percent lumps afterpreexpansion. The preexepanded beads were permitted to age for 24 hoursthen were molded into a /2 x x 5 inch block by placing the moldcontaining the beads between the platens of an electrically heatedpress. The foam block had a density of 0.99 lbs/cu. ft. and the fusionwas excellent. The foam block was cut into 5 one inch strips and thestrips conditioned overnight in a 5060 oven to remove any residualblowing agent. The foam strips were suspended vertically in a draft-freehood and ignited by holding a /2 inch flame from a microburner incontact with the bottom of each strip for from 35 seconds. The averagetime to extinguishment of sustained burning after the flame was removedfrom the strips was determined. The average time to extinguishment ofthe 5 strips was 1.0 seconds. An average time to extinguishment of 1.0second or less is considered to adequate for commercial acceptability.

While the foregoing examples utilize styrene monomer as thepolymerizable vinyl aromatic monomer it is to be understood that othervinyl aromatic monomers can be employed in the process and product ofthe invention and the term thermoplastic polymers as used hereinincludes a variety of homopolymers and copolymers derived from vinylaromatic monomers including styrene, divinylbenzene, isopropyl styrene,alpha-methyl styrene, nuclear dimethyl styrene, chlorostyrene, vinylnaphthalene, etc, as well as polymers prepared by the copolymerizationof a vinyl aromatic monomer with monomers such as butadiene andacrylonitrile wherein the vinyl aromatic monomers present in at least 50percent by weight.

The foregoing has described novel expandable polymers and their methodof preparation whereby the problem of lumping on preexpansion has beensolved without the heretofore encountered difficulties of otheranti-lumping systems which harm the fusion properties of the beads tothe point where it has not been possible to obtain commerciallyacceptable moldings.

We claim:

1. A method of preparing expandable thermoplastic polymer particleswhich will not lump upon pre-expansion comprising polymerizing a vinylaromatic monomer, either alone or in admixture with other polymerizablemonomers copolymerizable therewith, in an aqueous suspension in thepresence of a catalytic amount of a free radical initiator and asuspending agent until between about 50 and 95 percent conversion of thevinyl monomer to polymer has occurred, then adding at least about 0.05percent by weight of an unsaturated polyester resin based on the Weightof said monomer, completing said polymerization, and rendering thepolymer particles expandable by incorporating therein from 320 percentby weight of a blowing agent.

2. The method of claim 1 in which said unsaturated polyester resin isadded between about and percent conversion of said monomer.

3. The method of claim 1 wherein a dispersing agent is mixed with saidunsaturated polyester resin prior to the time that said resin is addedto said suspension.

4. The method of claim 1 wherein said unsaturated polyester resin is thecondensation product of propylene glycol with maleic or phthalicanhydride and mixtures thereof.

5. A method of preparing expandable styrene polymer particles whichcomprises polymerizing a vinyl aromatic monomer in an aqueous suspensionin the presence of a catalytic amount of a free radical initiator untilbetween 50 and 95 percent of said monomer has been converted to polymer,then adding at least 0.05 percent by weight of an unsaturated polyesterresin based on the weight of said monomer, completing the polymerizationof the monomer and rendering the polymer expandable by incorporatingtherein from 3-20 percent by weight of a blowing agent.

6. An expendable styrene polymer particle made by polymerizing a vinylaromatic monomer in an aqueous suspension in the presence of a catalyticamount of a free radical initiator until between 50 and 95 percent ofthe monomer has been converted to a polymer, then adding at least about0.05 percent by weight of an unsaturated polyester based on the weightof said monomer, completing said polymerization and incorporating in thepolymer from 3-20 percent by weight of a blowing agent thereby producingpolymer particles which will not lump upon preexpansion.

References Cited UNITED STATES PATENTS 2,865,800 12/1958 Stastny260--2.5

MURRAY TILLMAN, Primary Examiner.

MORTON FOELAK, Assistant Examiner.

1. A METHOD OF PREPARING EXPANDABLE THRMOPLASTIC POLYMER PARTICLES WICHWILL NOT LUMP UPON PRE-EXPANSION COMPRISING POLYMERIZING A VINYLAROMATIC MONOMER, EITHER ALONE OR IN ADMIXTURE WITH OTHER POLYMERIZABLEMONOMERS COPOLYMERIZABLE THEREWITH, IN AN AQUEOUS SUSPENSION IN THEPRESENCE OF A CATALYTIC AMOUNT OF A FREE RADICAL INITIATOR AND ASUSPENDING AGENT UNTIL BETWEEN ABOUT 50 AND 95 PERCENT CONVERSION OF THEVINYL MONOMER TO POLYMR HAS OCCURED, THEN ADDING AT LEAST ABOUT 0.05PERCENT BY WEIGHT OF AN UNSATURATED POLYESTER RESIN BASED ON THE WEIGHTOF SAID MONOMER, COMPLETING SAID POLYMERIZATIION, AND RENDERING THEPOLYMER PARTICLES EXPANDABLE BY INCORPORATING THEREIN FROM 3-20 PERCENTBY WEIGHT OF A BLOWING AGENT.